Optical transmitters and receivers using polarization multiplexing

Disclosed is a transmitter that modulates a single-wavelength laser signal with multi-level amplitude modulation on each of two polarizations, with an additional multi-level inter-polarization phase modulation. In an experimental setup, four-level amplitude modulation is used on each of the two polarizations, and four-phase inter-polarization phase modulation is used. Other numbers of levels may be used, in variations of the disclosed techniques and apparatus. Also disclosed is a corresponding receiver, which includes a DSP algorithm that recovers, simultaneously, the information on the multiple intensities imprinted by the transmitter on each polarization and the information from the multi-level inter-polarization phase modulation.